This invention relates generally to diesel engines and, more particularly, to medium speed diesel engines for operation at high altitudes.
In a diesel engine, liquid fuel is injected into a plurality of engine cylinders full of compressed air at high temperature. The fuel is broken up into droplets, which evaporate and mix with the air in the cylinders to form a flammable mixture. Since proper combustion in the cylinders requires full oxidation of fuel though evaporation, species diffusion, and mixing with air, the amount of cylinder-charged air, or air to fuel ratio of the mixture plays an important role in diesel engine fuel-air mixing and combustion, which, in turn, affects fuel efficiency and exhaust emissions. This is particularly true for quiescent chamber-type medium speed heavy-duty diesel engines where the cylinder air intake swirling is slight, such as locomotive or marine type engines having cylinders with relatively large displacement volumes. The fuel injection timing of medium speed diesel engines at full load is typically set so that the peak firing pressure in the cylinders is at or below a maximum cylinder firing pressure under normal altitude operation, i.e., at about sea level.
Engine exhaust emissions, including carbon monoxide (CO), particulate matters (PM) and smoke, are generated when the air-fuel mixture is incompletely combusted. When engines are operated at higher altitudes, i.e., at a low barometric pressure, lesser amounts of air are introduced into the cylinders, causing the air-fuel mixing process to deteriorate relative to lower altitude, higher ambient pressure environments. This combination of factors increases untimely and incomplete combustion in the engine cylinders which lowers fuel efficiency and increases exhaust emissions of CO, PM, and smoke. The reduced amount of air for the fuel-air mixture combustion, together with the increased untimely combustion, typically leads to increased cylinder exhaust gas temperatures. For engines including a turbocharger, the decreased barometric pressure and the increased exhaust temperature cause an increase in turbocharger speed. This usually requires power duration to prevent turbocharger damage from overheating and excessive speed.
Accordingly, it would be desirable to operate a diesel engine in a manner that avoids the resultant increase in exhaust emissions while operating at higher altitudes. Additionally, it would be desirable if a diesel engine could be operated at high altitudes with minimal deterioration of engine efficiency and power capacity, and without sacrificing the engine performance at normal altitude operation.